1. Field of the Invention
The present invention relates to a manufacturing method of a rack shaft having a rack teeth in a part of a peripheral surface.
2. Description of Related Art
A rack shaft is manufactured by a rack cutting machine or a forging. The manufacturing method by the forging uses a lower mold having a receiving groove, and an upper mold having a punch at a position facing to the receiving groove, and forms a rack teeth by mounting a shaft raw material on the receiving groove, thereafter executing a die matching of the upper mold and the lower mold, linearly moving the punch having a plurality of rack-teeth-forming teeth portions, and pressing the shaft raw material in a diametrical direction (See, for example, Japanese Patent Application Laid Open No. H04-28582). Further, as the manufacturing method by the forging, there has been known a manufacturing method of using an upper mold having a plurality of rack-teeth-forming teeth portions without using the punch, and linearly descending the upper mold so as to form the rack teeth on the shaft raw material.
However, in the method of manufacturing the rack shaft by linearly descending the punch or the upper mold, since the pressing force of the punch or the upper mold is applied to the peripheral surface of the shaft raw material all at once, it is hard to compress the shaft raw material into an entire region of a bottom of the rack-teeth-forming teeth portion. Further, since the pressing force of the punch or the upper mold becomes strongest in a center portion of the rack-teeth-forming teeth portion, and the pressing force becomes weaker in accordance with being back away from the center portion, an amount of deformation in a diameter direction (a direction of a face width) is smaller in comparison with an amount of deformation in a direction of an axial length. Accordingly, in the method of manufacturing the rack shaft by linearly moving the punch or the upper mold, there is a problem that an underfill tends to be generated in an end of the rack teeth in the direction of the face width, and it is hard to form an accurate rack teeth.